Ralphs Bay is a body of water in south-east Tasmania, Australia. It is semi-enclosed by the Tranmere / Rokeby peninsula and the South Arm peninsula. Sea access to the bay is from the River Derwent. Ralphs Bay is a shallow, windy bay, situated on the south-east of the Derwent Estuary, about 12 kilometres from the south-east of Hobart in Tasmania, Australia. The bay is sheltered by the low, grassy hills of Droughty Point to the north-west and the spit of South Arm and Opossum Bay to the south-west.